Treatment of metalworking tools



Patented Nov. 20, 1951 UNITED STATES PATENT OFFICE TREATMENT OFMETALWORKING TOOLS Edward T. Pavlock, Detroit, Mich.

No Drawing. Application July 9, 1946, Serial No. 682,285

3 Claims. 1

The present invention relates to methods of treating metal workingtools, including those having cutting edges, to create a hard surfacecase of greater depth and having greater toughness, durability andresistance to corrosion than the cases developed by prior methods, inorder to increase the useful life of such tools. It is applicable totools of all types made from high speed steel and other compositions oralloys of steel which are suificiently hard to perform the actions onmetal work pieces for which they are designed. The procedure in which itis embodied follows the completion of the heat treatingprocessesheretofore employed in the manufacture of such tools. It may be, andpreferably is, performed on completely finished tools, that is, thosewhich have beenshaped to final form and, in the case of cutters, thoseof which the cutting edges have been sharpened. However, the moregeneric aspects of the invention include employment of the treatment atearlier stages of manufacture than those indicated.

The following steps constitute the preferred embodiment of theinvention.

First, oil or grease adhering to the tool or other article to be treatedis removed. Any degreasing treatment or substance suitable for thepurpose may be used in this step.

Second, the article is preheated to a temperature of 800? F. orapproximately that temperature. This heating step may be performed in abox type electric furnace wherein the temperature is maintained at theprescribed degree, and the work is held in the furnace until it hasacquired the temperature of the furnace.

Third, the work is immediately transferred to a furnace, for instance apot furnace, in which a higher temperature is maintained and in which itis subjected to chemical reaction with nitrogen to form a nitride caseon its surfaces. The temperature of nitriding treatment is from 1025 F.to 1050 F. and the period of exposure to the nitriding treatment ispreferably fifty minutes regardless of the dimensions and design of thetool. In this step the exposure of the tool to nitrogen may beaccomplished by covering the tool with a cyanide or mixture of cyanides.Any commercially prepared so called hard casing salt may be used here,but best results have been obtained in practice with the use of a bathcomposed of 60% sodium cyanide and 40% potassium cyanide. However, anyfeasible mode of enveloping the tool in a nitriding atmosphere or bath,while maintaining the temperature within the effective range for thedesired length of time may be used.

It is important that, in the nitriding step, nickel beexcluded,.wherefore the salt bath is contained in a nickel-free pot anda nickel-free thermo couple is used.

Fourth, the work is washed thoroughly to remove salt. It is importantthat all, or as much as possible, of the salt adhering to the surfacesof the work be removed.

Fifth, the work pieces are packed in sheet steel boxes with a granularpacking material to exclude air during the prolonged heating whichfollows in the next step. Granulated charcoal which will pass through ascreen of A," mesh is a suitable packing material, although not the onlyone that can be successfully used. The preferred mode of packing is toplace a layer of charcoal about deep in the bottom of the box, then laya number of tools to be treated on the packing material, as many as canbe accommodated in the area of the box without touching each other, thento cover the articles with another layer of charcoal deep; and repeatuntil the last placed layer of tools comes to within about 1" of the topof the box, and to fill the remaining space with charcoal.

Sixth, the box and the work pieces therein are subjected to prolongedheating treatment at a temperature near that at which the steel of whichthe work pieces are made commences to soften, and preferably in exposureto an oxidizing medium. In the case of tools made of high speed steelthe optimum temperature is 1025? F. or very near that temperature. Itshould be as near as possible to the drawing temperature.

of the steel but not so high as to cause appreciable softening. Withother alloy steels other specific temperatures may be employed, butalways in accordance with the considerations above set forth. The heatsoaking treatment is continued for approximately three hours for alltools of one inch or less in thickness and for an additional three hoursper additional inch of thickness.

A suitable mode of carrying out the step last described is to place thebox containing the nitrided tools in an electrically heated furnacehaving conduits by which water can be conveyed into and discharged at arelatively high level in the chamber wherein the tools are contained.When the tool or tools undergoing treatment have been brought to thetemperature of the furnace, Water is admitted to the delivery conduitsand allowed to flow at a rate which causes discharge of approximatelydrops per minute for every two cubic feet of internal furnace space. Anoxidizing effect is produced thereby.

Similar effects may be obtained by covering the work pieces with a socalled neutral salt which has oxidizing properties.

This heat treatment seems to diffuse the concentration of nitrides, thusdecreasing the brittleness of the hard case, making it tougher andstronger. It also drives the nitride layer deeper into the tool. Thislast effect is aided and enhanced by the oxidizing reaction. Theoxidized surface also resists attack by moisture and other agencieswhich cause rusting of steel.

Seventh, the box with its contained workpieces is removed from thefurnace and allowed to cool for about 30 minutes. If the packingmaterialused is charcoal or other inflammable substance, the top layer isbrushed off immediately in order to prevent it from igniting. Afterpreliminary cooling in the box, the work pieces are removed and allowedto cool in still air.

Preferably the tools are pickled by immersion in abath of acidlongenough to remove any fuzzy oxide which may have been formed.- onthe-tools during the preceding heat treating operation. It has beenfound that a bath of 18 commercial muriatic acid gives verysatisfactoryresults; although I am not limited to the use-of thisparticular acid. The pickling step seems to addmaterially to the life ofthe tools.

The acid is then washedoff with warm water; and'a thin blacksu-bstancewhich may have adhered to the tool is removed by brushing or theemployment of any other means that will serve thepurpose withoutdamaging the'cutting edges of sharpened edged tools. Then the tools aredried, preferably by an air blast.

Finally the tools may be coated with oilor any other suitable rustpreventive.

The treatment hereindescribed has been found toincrease the useful lifeof cutting tools in very large measure; up to an increase of more than200 in many cases. Of the steps described, those ofnitriding andprolonged heatingat a hightemperature, which is not high enough tosoften the metal, are vitally important. The pickling step also isbelieved to have a substantial iniiuencein giving prolonged life, vas ithas beenobserved that under similar conditions tools which haveundergone. this. step remain serviceable longer than those which havenot.

While I have namedcertain specific valuesand figures in the foregoingspecification, I Wish to point out that the scope of the invention whichI desire to protect hereby includesall variations and departures fromthose specific values which obtain equivalent results.

What I claim is:

1. The method of case hardening a steel tool which comprises preheatingthe tool to a temperature of 800 F., holding the heated tool immersedfor a period of about 50 minutes in a nitriding bath maintained at atemperature of from 1025 to 1050 F., cleaning the tool, packing the toolin granulated carbon, maintaining the packed tool for at least threehours in an enclosed space at a temperature near to, but below,

the hardness-drawing temperature of the steel,

and admitting water continuously at a regulated slow rate to suchenclosed space.

' 2. The method of case hardening a tool of high speed steel whichcomprises preheating the tool to a temperature of 800 F., immersing theheated tool for a period of about 50 minutes in a nitridingc'yanide bathmaintained at a temperature of from 1025 to 1050 F., cleaning the tool,packing the tool in-granulated'carbon, maintaining the packed tool forat least three-hours in an enclosed space at a temperatureofapproximately 1025 F., and admitting Water to such enclosed space-aterate ofapproximately drops per minute per2 cubic feet of suchenclosedspace.

3. Themethodof modifying-the characteristics of anitridedcaseona highspeed steeltool which comprises packing the tool in granulatedcarbon,maintaining the packed tool for at least-three hours in an enclosedspace at a temperature of approximately l025 :E, and continuously.admitting water to such enclosed-space at a regulated slow rate ofapproximately 120 drops per minute for each 2 cubic feet of the enclosedspace.

EDWARD TrPAVLO CK.

T001 Steels, Gill .et .al., .1944, Soc. for Metals, pp. 485-489.

Number Molten SaltBaths," E. I. du Pont de Nemours and 00., 1942, page27...

Transactions of The American Society for Metals, vol. 27, .l939.,.page1000.

The Journal of the Iron. and .Steel Institute, vol. 136, No.2, 1937 pp.153-158.

1. THE METHOF OF CASE HARDENING A STEEL TOOL WHICH COMPRISES PREHEATINGTHE TOOL TO A TEMPERTURE OF 800* F., HOLDING THE HEATED TOOL IMMERSEDFOR A PERIOD OF ABOUT 50 MINUTES IN A NITRIDING BATH MAINTAINED AT ATEMPERATURE OF FROM 1025* TO 1050* F., CLEANING THE TOOL, PACKING THETOOL IN GRANULATED CARBON, MAINTAINING THE PACKED TOOL FOR AT LEASTTHREE HOURS IN AN ENCLOSED SPACE AT A TEMPERATURE NEAR TO, BUT BELOW,THE HARDNESS-DRAWING TEMPERATURE OF THE STEEL, AND ADMITTING WATERCONTINUOUSLY AT A REGULATED SLOW RATE TO SUCH ENCLOSED SPACE.